1. Field of the Invention
The present invention relates to a wall flow type exhaust gas purification filter. More particularly, the present invention relates to a wall flow type exhaust gas purification filter suitably used for purifying of particulate matters and noxious gas components such as nitrogen oxide (NOx), carbon monoxide (CO), and hydrocarbon (HC) especially contained in exhaust gas from automobile engines.
2. Background Art
Reduction of fuel consumption by automobiles has been demanded in recent years from the viewpoints of influences on the global environment and resource saving. This leads to the tendency to use internal combustion engines having good heat efficiency such as a direct-injection gasoline engine and a diesel engine more as a power source for automobiles.
These internal combustion engines, however, have a problem of cinder generated during the combustion of fuel. Considering the atmospheric environment, countermeasure is required not to release particulate matters (hereinafter this may be called “PM”) such as soot and ashes to the atmosphere while removing noxious components from the exhaust gas.
Regulations on the removal of the PM exhausted from a diesel engine have been made especially tighter on a global basis, and so a honeycomb-structured wall flow type exhaust gas purification filter attracts attention as a collection filter to remove the PM (hereinafter this filter may be called a “DPF”), and various systems have been proposed. The DPF is typically configured to include a plurality of cells as through channels of fluid that are defined and formed by a porous partition wall, where the cells are plugged alternately, whereby the porous partition walls defining the cells serve as a filter.
The DPF is configured to let in exhaust gas or the like containing particulate matters from a first end face (inflow-side end face) to filter the particulate matters with the partition walls, and to let out the purified gas from a second end face (outflow-side end face). Such a DPF has a problem of the particulate matters contained in the flow-in exhaust gas being accumulated on the partition walls, causing the clogging of the inflow-side cells. This often happens in the case of containing a lot of particulate matters in the exhaust gas or in cold climate areas. Such clogging of the cells leads to the problem of abrupt increase in pressure loss at the DPF. Then to suppress such clogging of the cells, the DPF is devised to increase the filtration area and the opening ratio at the inflow-side cells of the exhaust gas.
Specifically, one proposed structure has different cross-sectional areas between the inflow-side cells, i.e., the cells that are open at the inflow-side end face (inlet opening cell) and the outflow-side cells, i.e., the cells that are open at the outflow-side end face (outlet opening cell)(hereinafter this may be called a “High Ash Capacity (HAC) structure”) (see Patent Document 1, for example). Herein, the cross-sectional area of a cell refers to the area of a cross section obtained by cutting the cell at a plane perpendicular to the central axis direction.
Another proposed honeycomb filter has such a HAC structure including inflow-side cells having a large cross-sectional area and outflow-side cells having a small cross sectional area, while having different cross-sectional shapes between the inflow-side cells and the outflow-side cells (see Patent Document 2, for example). Herein, the cross-sectional shape of a cell refers to the shape of a cross section obtained by cutting the cell at a plane perpendicular to the central axis direction.    [Patent Document 1] WO 2009/069378    [Patent Document 2] JP-A-2004-000896